1. Field of the Invention
The present invention relates generally to a fluorescence detecting device for detecting a fluorescence reaction, for instance, a fluorescence detecting device suitable for detection, etc., of a specific gene contained in a sample.
2. Related Background Art
Recently, the genome sequence analysis has been developed significantly, and the determination of the whole base-sequence of the human genome will be completed in 2003. Besides, the determination of genomes of other creatures is proceeding throughout the world. With this development of the genome analysis, the detection of genes has an increased significance from the viewpoint of the determination of functions of genes, the medical diagnosis, etc. Examples of conventional gene detecting methods include the gene amplification methods represented by the polymerase chain reaction (PCR) method, while recently the gene detecting method employing DNA chips is used widely.
A DNA chip is an approximately 1 cm×1 cm glass chip, silicon chip, etc. on which a plurality of single-strand DNAs are fixed. Examples of the single-strand DNAs to be fixed include DNAs as etiologic genes. The gene analysis employing a DNA chip is performed, for instance, in the following manner. First of all, a target gene is extracted from cells (for instance, blood cells). Then, the target gene is amplified by the PCR method. In the amplification, a fluorescent substance is employed to label an amplification product. A DNA chip is immersed in a solution containing nucleic acid strands labeled with the fluorescent dye, so that hybridization occurs. Thereafter, the DNA chip is washed so that nucleic acids that have not been hybridized are removed.
Subsequently, the DNA chip is irradiated with an excitation light, and the fluorescence is detected. An example of a fluorescence detecting device used herein is shown in FIG. 11. In the device, an excitation light 309 from a light source 305 such as a laser is reflected by a beam splitter 304, and enters an objective lens 306, where the light is focused so as to be incident on a fixed portion 307 of a nucleic acid probe on a DNA chip 308. In the case where a double strand is formed as a result of hybridization, a fluorescent substance is present on the DNA chip 308, and therefore, a fluorescence 310 is emitted upon the irradiation by the excitation light 309. Normally, the fluorescence 310 and the excitation light 309 have a wavelength difference on the order of several tens of nanometers. A part 311 of the fluorescence and a reflected light of the excitation light 309 return to the objective lens, and reach the beam splitter 304. Most of the reflected light of the excitation light 309 is reflected by the beam splitter 304, thereby being directed to the light source side. The part 311 of the fluorescence passes through the beam splitter 204, thereby being directed to a photodetector 301. The part 311 of the fluorescence that has passed through the beam splitter 304 passes through a filter 303 that limits a wavelength, while the reflected light of the excitation light 309 is blocked by the same. Furthermore, the part 311 of the fluorescence passes through a photodetector lens 302 and enters the photodetector 301 for measuring an intensity of the fluorescence, where the fluorescence is detected.
However, the above-described conventional fluorescence detecting device is a large-scale and complex device having a long optical path, through which the fluorescence is lost partly, thereby leading to a problem of low detection sensitivity.